Insulated wires are often used when contacting electrical components. In many products, the trend toward miniaturizing components leads to large savings in terms of material. Due to the small dimensions, the miniaturized components can generally be produced in panelized form; a large number of components are therefore manufactured in parallel on a carrier, resulting in huge cost savings. However, miniaturization also primarily enables the development of new applications through miniaturized components and assemblies.
In medical technology, for example, blood pressure can be determined directly in the coronary vessels by means of very small sensors. Miniaturized actuators are also suitable to be used for transmitting and in turn receiving sound waves in the coronary vessels. Thus, the flow rate of the blood is able to be determined via the shift in frequency. In addition, during catheterization, the reaction force of instruments in the coronary vessels is able to be measured by means of miniaturized force sensors. Further uses of miniaturized electrical components can be found in the aerospace industry, motor vehicle engineering, building services engineering, and the consumer goods industry, as well as in almost all areas of technology in the future.
Microsensors often have to be supplied with energy, and the signals have to be conducted over several meters. For that purpose, electrical wires, often microcables, are advantageously used, particularly in the case of connection lengths greater than 60 cm. In order to protect these wires from short-circuiting with one another and from external influences and to ensure mechanical protection, these wires may be configured in such a way as to be insulated from one another. For many applications, a particularly stable, fully closed insulation is required which suppresses any flow of current, even in a moist environment.
Nowadays, standard methods of semiconductor technology exist for producing electrical microcomponents. The production of electrically stably insulated wires is also state of the art, for example in the production of wires for high-frequency technology which may comprise diameters of less than 20 μm and which are able to be produced inexpensively from copper, and thus also have a very high conductivity. At present, however, no method exists which allows the reproducible, inexpensive contacting of stably insulated, inexpensive wires on microcomponents. This problem is solved for the first time by the method of the present disclosure.
In order to electrically contact insulated wires, a region which is not insulated but which is instead easily able to be electrically contacted has to be provided at the contact point.
One known contacting method which is used in microengineering is ultrasonic wire bonding, which represents the standard for the cold welding of microwires. It is used more rarely for contacting microwires with lengths greater than 60 centimeters, since the wire feed generally takes place through a very thin capillary which is not optimized for the pulling of the wire. In general, wire bonding is used for short contacting distances from the chip to the chip carrier. More recent efforts are aimed at the bonding of electrically insulated wires. This results from the increasingly small contact spacings and the lower bending stiffness of thinner wires, so that short-circuits could occur due to a component falling over before plastic has been injected around it. The mechanical stability of the insulating material used for these bondable wires is configured such that it is easily able to be penetrated by the bonding tool. Alternatively, the insulation is able to be removed or largely damaged by flaming it off the wire, and a contact is able to be established by bonding. Furthermore, the insulation only has to insulate a contact point with surface areas in the range of a few square nanometers, namely the area of two randomly overlapping wires, and under an extremely low mechanical load. These bonding methods are therefore not suitable for contacting inexpensive, stably insulated copper wires on microcomponents. A further increase in the bonding energy, as proposed in DE 16905038 A1, may lead to the component being destroyed and is often not suitable for sensitive semiconductor chips. A contacting of balls formed via flames or sparks, as proposed in DE 3642221 C2, is likewise not easy to carry out at present with copper wires and, due to the small process window, is not part of the frequently used state of the art. In both methods, there is the disadvantage that in each case the insulation has to be removed in a defined manner by applying energy without destroying the wire or the chip. With the stable insulations required for this and the precision of the required contact point, this is hardly ever possible and residues of the insulation remain in the contact region. The currently available wires which would be suitable for microcontacting do not comprise the necessary stability in terms of the mechanical abrasion resistance of the insulation, do not have a fully closed and therefore waterproof insulation, and are too expensive. The metals used are designed for contacting by means of ultrasonic bonding and thus comprise a ductility which opposes a tensile strength of the wire that is as high as possible, though the latter is nevertheless desirable and necessary for easy and inexpensive handling of long microwires.
Another method for contacting electrical wires on microcomponents is electrically conductive gluing. In this case, the surfaces of the contact point are finished (e.g. with a gold coating) in order to obtain a low transfer resistance which is stable for a long time. The dosing of the gluing or soldering auxiliary in the nanoliter range is the difficult part of this process. Application by means of microdispensers is very complicated since an alignment with each contact is necessary. Another possibility would be to apply the contact auxiliary to the wire end which is to be contacted. However, due to adhesion forces, the contact auxiliary usually spreads a little along the wire, meaning that not enough contact aid remains in the actual contact region.
Furthermore, it is difficult to create a defined contact region at the wire end. Possibilities include the use of finished and insulated copper wires or other finished metal wires. These are cost-intensive in terms of the basic price due to the high content of noble metal in the coating and base material and the low margins for special applications. In addition, a necessary removal of the insulation in the contact region in turn requires a very precise individual manipulation. For instance, the wire underneath the insulation could be exposed by means of a microblade, laser ablation or the application of local heat or of solvent. This is complicated and cost-intensive and, particularly in the micro range, is extremely difficult to carry out and is a source of error. For example, EP 1 396 915 A1 describes a method for stripping insulation from cable bundles by means of two different laser sources. In this case, the first insulation step is to be carried out using a long-wave laser so that a material thickness in the micrometer range remains, which in turn is removed using a short-wave laser. These methods are complicated and it is difficult to achieve a stripping of the insulation to micrometer accuracy along the length of the wire, and the insulation has to be suitable for laser machining, accordingly, and has to be available in a homogeneous thickness.
A defined contact region is also able to be created by weakening the insulation, thus generating a predetermined breaking point, and exposing a defined wire region by shifting the insulation to one side and then coating said wire region, as disclosed in DE 33 12 190 C1. With a required stable insulation on an individual conductor, this cannot be used or can be used only to a very limited extent since a very adhesive coating cannot be shifted. Furthermore, the positioning, which is accurate to within a few micrometers and is achieved by shifting the insulation, can only definitely be achieved with extreme difficulty due to the necessary micromanipulators and therefore cannot reasonably be used.
The methods from the state of the art are therefore not suitable for creating a precisely defined contact region on the micrometer scale, the production of which is virtually independent of the parameters of the insulation. Moreover, they do not solve the problem of dosing a contact auxiliary onto a noble contact system at the wire tip in the same working process and with the same accuracy as disclosed below.